docs: def push_func(self, cuin, callback):
          """Push a function for dfp.
  
          :param cuin: str,unicode: Callback Unique Identifier Name.
  
          :param callback: callable: Corresponding to the cuin to perform a function.
  
          :raises: DFPError,NotCallableError: raises an exception
  
          .. versionadded:: 2.3.0
          """
          if cuin and isinstance(cuin, string_types) and callable(callback):
              if cuin in self._dfp_funcs:
                  raise DFPError("The cuin already exists")
              else:
                  self._dfp_funcs[cuin] = callback
          else:
              if not callable(callback):
                  raise NotCallableError("The cuin %s cannot be called back" % cuin)
              raise DFPError("Invalid parameter")